If you are talking about what happens in Section 3, then the answer is that Montag finds (or is found by) a group of men who call themselves "book covers." They are led by a man named Granger.
These book covers are people who have decided to leave the society behind and go out to the country. They cannot stand the way that people are forced to live in the society. Each of the men has memorized a book or part of a book. They are hoping to keep those books "alive" until something happens that will let them come back to society and try to help it. They hope to remake society some day using the ideas from the books that they have memorized.